This invention relates generally to methods and apparatus for sealing and more specifically relates to methods and apparatus for sealably coupling tubular members such that an occlusive seal is formed in response to pressure from a first direction, but that the seal will vent pressure from a second direction.
In many applications, and particularly in applications commonly found in the oil and gas industry, it is necessary and desirable to couple tubular members together such that an occlusive seal is formed and will be maintained, even under conditions involving severe temperatures and pressures. In the oil and gas industry, this need typically arises with regard to well casing and tubing which typically must be capable of achieving such sealing through multiple cycles of coupling and uncoupling of the members.
Several mechanisms have been used by the prior art to attempt to provide the characteristics described above. Oilfield tubular members, such as well casing and tubing typically have a member having a male thread or pin end designed to engage an axially aligned tubular member having a female thread or box end. It is known in the art to seal between the members by using a metal-to-metal seal. Often, the box end will have a section exhibiting a frusto-conical aperture profile. The pin end will then have a section having a generally frusto-conical profile except that the surface will have an axial peripheral bow, causing the section to exhibit a convex profile which is non-complementary to the corresponding seal surface of the box end. When the pin and box ends are threadably coupled to the designed thread stop, the convex surface of the pin engages the surface of the frusto-conical section of the box, forming a point-of-contact metal-to-metal seal. To achieve the desired sealing the mutually-engaging surfaces of the pin and box must be smooth and true, and are often highly polished to achieve this end.
Another form of metal-to-metal seal exhibits complementary abutting shoulders on the pin and box such that the shoulders engage one another upon threaded coupling, often providing a thread stop for the coupling. Again, engaging surfaces of the seal must be in extremely smooth condition to effect a suitable seal.
As the pressures to which such a seal is subjected rise, the criticalness of the condition of the contact surfaces increases. It is not uncommon for damage to occur to the engaging seal surfaces of the tubular members, particularly to that of the pin member, through contact during handling or during stabbing of the members prior to rotation for engagement. Metal-to-metal seals are also often susceptible to damage resulting from corrosive environments which attack the surface integrity of the seals. Further, metal-to-metal seals often vent to pressure from a secondary direction at a lower pressure than that at which they sealed in response to pressure from a primary direction. After this venting, the seals do not necessarily return to sealing against fluid pressures from the primary direction equal to those which were originally sealed against.
In an attempt to cure some of these deficiencies, the prior art has supplemented the metal-to-metal seals with deformable plastic seals adapted to be compressed between the two members and seal therebetween. These plastic seals provide a secondary or backup seal to the metal-to-metal seal. The secondary plastic seals may be situated adjacent the metal-to-metal seals or may be placed in grooves adjacent the thread coupling or in the thread coupling itself.
Several difficulties are presented by the use of these plastic seals. Initially, because the seals are compressed directly between two surfaces, the seals typically vent at equal pressures from both primary and secondary directions, thereby effectively sealing only up to a threshold level at which the seal vents. A major difficulty with the plastic seals is that they typically have a thermal coefficient of expansion which is 6 to 10 times that of the steel parts with which they are associated. Two significant problems thus result. First, as temperature rises, the plastic seals will expand to an extent much greater than will the tubular members. Therefore, the plastic seals act as a wedge, actively forcing the metal members apart. This can cause not only a reduction or elimination of the integrity of the metal-to-metal seals, but can also separate the thread annulus such that the tensile efficiency of the connection is reduced. Secondly, where the secondary plastic seals are situated such that a fluid leaking past the primary seal may enter the thread annulus, because of the essentially non-venting characteristics of the plastic seals, the fluid may be trapped within the thread annulus where it may expand and again weaken the integrity of the threaded coupling.
Accordingly, the present invention provides a method and apparatus for forming a self-energizing seal between two tubular members, such seal engaging in response to pressure from a primary direction while venting to pressure from a secondary direction, thereby maintaining the integrity of the metal-to-metal seal and thread annulus.